The projects listed below are combined by their focus on erythrocyte transglutaminase, a post-translationally acting protein modifying enzyme, capable of the rapid cross-linking of some of the membrane skeletal monomeric proteins. The topics are subdivided into the following categories with the specific aim of understanding the expression and functioning of this red cell enzyme in its broadest context. I.Protein modifications in the membranes of pathological red cells. II.A novel, in vitro, model for the precipitation of Hb onto the membrane (Heinz body formation). III.Exploring transglutaminase-reactive contact domains of isolated human red cell membrane proteins. IV.Interfering with membrane protein cross-linking and proteolysis in the human red cell by novel inhibitors. V.Thrombosis without thrombin in hemolytic diseases. VI.Molecular biology of red cell transglutaminase.